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COMPARATIVE CACTUS ARCHITECTURE AND PAR INTERCEPTION
Author(s) -
Geller Gary N.,
Nobel Park S.
Publication year - 1987
Publication title -
american journal of botany
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.218
H-Index - 151
eISSN - 1537-2197
pISSN - 0002-9122
DOI - 10.1002/j.1537-2197.1987.tb08709.x
Subject(s) - cactus , phototropism , biology , shrub , interception , solstice , canopy , botany , equator , xanthium , photosynthetically active radiation , photosynthesis , ecology , physics , blue light , latitude , astronomy , optics
Because CO 2 uptake by cacti can be limited by low levels of photosynthetically active radiation (PAR) and because plant form affects PAR interception, various cactus forms were studied using a computer model, field measurements, and laboratory phototropic studies. Model predictions indicated that CO 2 uptake by individual stems at an equinox was greatest when the stems were vertical, but at the summer and the winter solstice CO 2 uptake was greatest for stems tilted 30° away from the equator. Stem tilting depended on form and taxonomic group; four barrel cacti in Ferocactus and in Copiapoa and four cylindropuntias in Opuntia tilted toward a horizontal light beam by an average of 18°, 48°, and 52°, respectively, after growth periods of 1 to 4 yr. In contrast, three columnar species showed no significant phototropic response, perhaps because structural stability requires their massive stems to be erect. Field plants of the dense, multiple‐stemmed shrub Opuntia echinocarpa had stems which tended to radiate outward from the plant base, and, although this would not influence the total PAR intercepted, it would result in a more uniform PAR distribution and hence higher CO 2 uptake. For O. echinocarpa and the even denser, mound‐forming Echinocereus engelmannii , PAR and chlorophyll decreased approximately exponentially with depth into the canopy. The canopies of O. echinocarpa and other cylindropuntias did not extend to the ground; in certain species, such truncation apparently resulted from a combination of very low PAR levels just below the lower canopy edge and the light‐dependent growth responses of individual stems. In addition, although the canopy surfaces of O. echinocarpa and O. acanthocarpa tilted toward the equator by about 30°, the canopies of other cylindropuntias tilted less or not at all; the computer model predicted that a 30° tilt would decrease interstem shading, increase daily PAR, and increase nocturnal CO 2 uptake by as much as 54, 26, and 24%, respectively. Not only can the shape of cacti be affected by PAR, but also shape influences PAR interception and hence CO 2 uptake.